1. Field of the Invention
This invention resides in the field of nanoparticle arrays, including nanotubes supported in thin membranes or sheets of polymeric material.
2. Description of the Prior Art
Nanotubes offer a wide range of applications that take advantage of their unique electrical, mechanical, and chemical properties. Arrays of nanoparticles, and specifically nanotubes, have been used in biomedical scaffolds, thermally and electrically conductive films, field emission devices, gas storage media, and nanofiltration membranes, including reverse osmosis membranes and ultrafiltration membranes. In many of these devices, the orientations of the nanoparticles in the array affect the device performance. Nanotubes can help disperse heat or electrical charges, for example, when the nanotubes are randomly oriented, and in some cases oriented in random directions within a plane. Conversely, nanotubes can promote the passage of molecules or energy in a given direction, such as across a plane, surface, layer, or film, when aligned in that direction. This effect is useful for example in nanofiltration membranes and reverse osmosis membranes, when most or all of the nanotubes traverse and extend fully through the membrane to serve as conduits for passage from one side of the membrane to the other. Water desalination membranes are examples of such membranes. The effectiveness of the inclusion of carbon nanotubes in these membranes arises from the fact that water molecules pass through nanotube channels at a significantly faster rate than salt ions or other species whose molecular size exceeds that of water. In some instances, the salt ions can be completely excluded from the nanotube channels. Nanotube-based water desalination media have been formed on silicon chips, as disclosed by Bakajin, O., et al. (Regents of the University of California), International (PCT) Patent Application Publication No. WO 2007/025104 A2, publication date Mar. 1, 2007, and by Holt, J. K., et al., “Fast Mass Transport Through Sub-2-Nanometer Carbon Nanotubes,” Science 312, 1034-1037 (19 May 2006). The fabrication process produces a dense, vertically-aligned array of double-wall carbon nanotubes (DWCNTs) on the surface of the chip by chemical vapor deposition (CVD), followed by the deposition of silicon nitride, also by CVD, to fill the gaps between the DWCNTs. Various other nanotube-containing structures designed for other uses also rely on transversely oriented nanotubes.